<h2>题目编号 : 215</h2>
<div style="color:#666;font-size:80%;">31 October 2008</div><br />
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<p>Consider the problem of building a wall out of 2<img src='images/symbol_times.gif' width='9' height='9' alt='&times;' border='0' style='vertical-align:middle;' />1 and 3<img src='images/symbol_times.gif' width='9' height='9' alt='&times;' border='0' style='vertical-align:middle;' />1 bricks (horizontal<img src='images/symbol_times.gif' width='9' height='9' alt='&times;' border='0' style='vertical-align:middle;' />vertical dimensions) such that, for extra strength, the gaps between horizontally-adjacent bricks never line up in consecutive layers, i.e. never form a &quot;running crack&quot;.</p>

<p>For example, the following 9<img src='images/symbol_times.gif' width='9' height='9' alt='&times;' border='0' style='vertical-align:middle;' />3 wall is not acceptable due to the running crack shown in red:</p>

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<img src="http://projecteuler.net/project/images/p_215_crackfree.gif" alt="" />
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<p>There are eight ways of forming a crack-free 9<img src='images/symbol_times.gif' width='9' height='9' alt='&times;' border='0' style='vertical-align:middle;' />3 wall, written W(9,3) = 8.</p>

<p>Calculate W(32,10).</p>




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